Shareware Grab Bag/Shareware Grab Bag.iso/007/dis86.arc/DIS86.DOC

Text File | 1986-08-30 | 20KB | 551 lines

            dis86 - Interactive 8086 Disassembler
                        James R. Van Zandt

SYNOPSIS

Dis86 is a full-screen, interactive disassembler of object code for the
8086, 8087, 8088, 80186, 80286, and 80386 (products of Intel), and the
V20 and V30 (products of NEC).  The 80386 disassemblies include 32 bit
operands and addresses.  Dis86 implements the concept of a "current
location" and allows use of the cursor keys to change it.  Code can
come from a .EXE file (in which case the header is properly
interpreted), any other file (assumed to have no header), or anywhere
in main memory (0000:0000 - F000:FFFF).  Dis86 can install changes,
even in an .EXE file, making it a convenient way to install patches.
Versions are available for the IBM PC (and clones) and Z-100.


STARTING THE DISASSEMBLER

To disassemble a file, give the file name (optionally preceded by a path
name) on the command line:

        A>dis86 foo.exe

To disassemble from RAM, use an empty command line:

        A>dis86

There are no command line switches.


FILE HEADER INFORMATION

For a .EXE file, the information in the file header will be displayed when
the program is first run and in response to the H command (see below).


DISPLAY SCREEN

During disassembly, the screen will resemble the following:

        0000:0100      e9 01 90            jmp            9104
        0000:0103      55                  push           bp
        0000:0104      8b ec               mov            bp,sp
        0000:0106      83 ec 0e            sub            sp,0e

                                  ...

        0000:012C      50                  push           ax
        0000:012D      b8 69 00            mov            ax,0069
        0000:0130      50                  push           ax
        0000:0131      e8 e9 5c            call           5e1d
        dis86 1.00 - A SHAREWARE software product  (c) 1986, James R. Van Zandt
        >
        ... 0000:0100  0000:0100  0000:0100

Lines 1 through 21 are the disassembled code.  Each line starts with
the current address, followed by the actual bytes being disassembled.
The rest of the line is the assembly language equivalent, if any, of
the code.  The display for A (ASCII), B (byte), and D (data) formats is
similar.  All numbers are shown in hexadecimal.

Line 22 is a message and prompt line showing, for example, the
arguments needed for some commands.  Line 23 has the prompt.  Typed
characters are echoed on the rest of this line.  Line 24 has three
addresses, which are the first three entries in the stack (see the
'cursor right' and 'cursor left' commands below).


CURSOR KEYS

The "current location" is the address displayed on the first line
of disassembly.  The cursor keys are used to adjust the current
location.

The up and down cursor keys (8 and 2 on the numeric pad) are used to
move the current location a small amount (note that they are not
inverses):

        <up>     moves up by one byte (lower address)
        <down>   moves down by one line (higher address)


The <pg up> and <pg dn> keys (9 and 3 on the numeric pad) move the
current location by larger amounts.  (These will not move the cursor
out of the disassembly buffer.  Otherwise, they are inverses.):

        <pg up>   moves up by 32 bytes (lower address)
        <pg dn>   moves down by 32 bytes (higher address)


The above keys change only the current location.  Other commands change
the current location by potentially large amounts, but first save it in
a stack.  The first three addresses in the stack are shown on the
command line at the bottom of the screen.

If the instruction at the current location is a jump, call, or a
reference to a data location, the cursor right key (6 on the numeric pad)
will push the current location on the stack and go to the referenced
location.  For a data reference, the disassembly format is changed to D
(hex and ASCII).

        <right>   follows a jump, call, or data reference


The cursor left or left arrow key (4 on the numeric pad) will pop the
last address off the stack.  Note that right arrow followed by left
arrow will return you to the same address, whereas left arrow
(returning, let us say, to address X) followed by right arrow will only
return you to the same address if there is an appropriate jump or call
at X.

        <left>    pops address stack

Aûfter using the right arrow or one of the commands A, B, C, D, or G (in
next section) to go to a new address, and using the left arrow key to
pop the stack, you will sometimes want to return to the previous
address.  The stack no longer holds the address.  However, the left
arrow key saves the current location in a special "previous state"
before popping the stack.

To return to the address stored in the "previous state", type shift
right arrow on a Z-100, or control right arrow on an IBM PC.

        <shift><right>   returns to "previous state"   (Z-100)
        <cntrl><right>   returns to "previous state"   (IBM)


In summary, the unshifted keys on the numeric pad are:

<home> top of file          ^   up 1 byte         <pg up>  up 32 bytes
                            |

<--    pop addr stack                             -->      follow jump/call

                            |
<end>  end of file          v  down 1 line        <pg dn>  down 32 bytes


<ins>  setup options

On the Z-100, the four keys with arrows on them may be used in addition
to the 2, 4, 6, and 8 on the numeric pad.


LETTER COMMANDS FOR MOVING THE CURSOR

There are five letter commands to change the display format and/or
disassembly address:

    A    ASCII data
    B    byte data (hex)
    D    data (both hex bytes and ASCII)
    C    code
    G    goto

These commands may be in upper or lower case.  Each may be followed by:

    <ret>        Only the display format changes.

    A <expression> <ret>
            The current location changes to the specified address.

    S <expression> <expression> <expression> <ret>
            The disassembler searches from the current
            address to the end of the buffer for the
            specified sequence of hex bytes.  If an
            expression has a segment specified using the
            ':' operator (below), the segment is ignored.

    S T [string] <ret>
            The disassembler searches from the current
            address to the end of the buffer for the
            specified ASCII string.  Cases are not
            distinct, and the high order bit is ignored.
            The string can also be introduced by a double
            quote.

    S R <expression> <ret>
            The disassembler searches from the current
            address to the end of the buffer for a
            reference (jump or call) to the specified
            address.

An <expression> can involve any of these items:

    hex numbers    (either upper or lower case letters)
    cs, ds, es, ss, fs, gs
            currently assumed segment register values
    $        current location
    @        offset of top address on the stack

...and any of these operators:

    + - * /        add, subtract, multiply, divide
    :        separate segment and offset

Note that G with no address is a noop.


OPTIONS

The 'O' command or <ins> (0 on the numeric pad) brings up menus for
changing setup options and allows the user to reset the disassembly
window.  Use <space> or <esc> to move to the next screen.

The first menu allows the user to select the processor which is
supposed to execute the code.  There is some conflict in op codes
between the V20 and V30 on one hand and the 80286 and 80386 on the
other.  That is, the two families use the same op codes for different
instructions.  Dis86 selects the instruction appropriate for the chip
shown in this menu.  In addition, instructions not implemented by the
indicated chip will be flagged.  The other item on the first menu lets
the user specify 16 or 32 bit mode for the 80386.  In the 16 bit mode
the 80386 is similar to the 8086.  In the 32 bit mode arithmetic is
performed in 32 bit registers and all address offsets are 32 bits.
(The 80386 itself selects the mode based on a bit in the segment table
entry for the code segment.)

The second menu allows the user to indicate the byte value which matches
any byte in a byte or character search (the "wild card" byte) and select
the number of bytes displayed on each line for the A, B, or D formats.
The latter value can also be set using the W command.

The last options display is a small map of the code being disassembled
which will resemble the following:

        ds= -10
        cs=0000
        |                  ss=0960
        es= -10            |
        | cursor=0000:0453 |
        CCCCCCCCCCCCCCcccccccccccccc
        ^0000:0000
                     ^0000:6144

The Cs represent the code being disassembled.  The capital Cs are the
portion of code in the disassembly window (see discussion below).  The
assumed values for the segment registers, the current location (labeled
"cursor"), and the beginning and end addresses of the disassembly
window are also shown.  The window can be adjusted using the right and
left cursor keys.

By entering the options menu with the <ins> key and stepping from one
menu to the next with <ret>, you can leave your right hand on the
numeric pad.


MISCELLANEOUS COMMANDS

The 'P' command is used to print a disassembly listing to a file.  The
first time this command is used, it prompts for a file name.  The
default file name is "printout".  To actually send the listing to a
printer, specify the filename "prn".  If the file already exists the
new information will be appended.  The file is automatically closed
before the disassembler exits.  The command also prompts for the
beginning and end addresses of the code to be printed.  The default
addresses print the current screen.  When the printing is finished, the
current address is advanced to the first byte not printed.  Thus, you
can repeat the sequence

        P <ret> <ret>

to print a large section.

Enter 'R' to display and/or change the assumed segment register values.
Entries may be full expressions.  For example, to copy the value from SS
into DS, use the cursor keys to select the DS register and type "ss".

The 'S' command selects a new segment register value for displaying
addresses.  The new register is shown on the message line.  The actual
address being disassembled is not changed (see "segmentation" below).

The 'W' command is used to set the number of bytes displayed on each
line for the A, B, and D formats.  This is useful for displaying
tables.  For example, when dis86 is executed without a file, it
displays bytes starting at address 0000:0000 and the width is set to
four so each interrupt vector is shown on a separate line.

Type '?' to get a series of help screens.  Type <esc> to return to the
disassembly, or any other key to advance to the next screen

The 'E' command allows the user to modify the program being
disassembled.  Changes are initially made only in the disassembly
buffer.  Before the buffer is overwritten or the disassembler
terminates, the user is asked whether the changes are to be written to
the file or RAM area being disassembled.

Enter 'Q' to stop the disassembler.


TYPING REQUESTED DATA

Many commands supply default entries for requested data.  If you decide
to accept the default, just enter <ret>.  For editing entries,
you can position the cursor using the left and right cursor keys to
move by one character, <home> (7 on the numeric pad) to move to the
left end of the string, or <end> (1 on the numeric pad) to move to the
right end.  Use the <del> or <backspace> keys to delete incorrect
characters, or just type characters to be inserted.  (There is no
"replace" typing mode.) In every case but one, you can also edit the
default entry by making <right>, <end>, or <del> your first keystroke.
The exception is the default for the byte search function.


DISASSEMBLY WINDOW

The disassembler uses a buffer to hold the code being disassembled.
For most purposes, this disassembly window is transparent to the user.
If the user requests an address within the file but outside the
disassembly window, the appropriate code is automatically read in.  The
existence of the window is apparent in only three cases:

    1.  If the disassembler is started near the end of the window and
        reaches the end before it fills the screen, the rest of the
        screen will be left blank.

    2.  The searches are done only from the current location to the end
        of the buffer.

    3.  If the contents of the buffer has been changed (see 'E'
        command) they are optionally written out before being
        overwritten.


LOAD ADDRESS

Code from a .COM file is displayed as though its Program Segment Prefix
were at 0000:0000 and its load address were 0000:0100.

Code from a .EXE file is displayed as though its load address were
0000:0000.  This puts its Program Segment Prefix is 10 paragraphs or
100 (hex) bytes lower.  This is somewhat awkward, because the DS and ES
registers are initialized to point to the PSP.  The disassembler
displays this segment value as -10.  The advantage of a load address of
0000:0000 is that no relocation is necessary.  The bytes displayed are
exactly the same as those in the file.  This also means that the code
can be modified (see below for the 'E' command) and written back to the
file without being "unrelocated".


SEGMENTATION

Addresses are displayed in segment:offset form, using the current
assumed value of the current segment register.  The current segment
register can be selected using the 'S' command to step among the
available registers (CS, SS, DS, ES, FS, and GS - the last two only
with 80386 code).  Changing segment registers or their values does not
move the disassembler cursor.  Only the displayed segment and offset
values will change to reflect the new assumptions.  Legal offsets will
be displayed as a four digit hex number (0000 to FFFF).  Other offsets
(negative or greater than 64K) will also be calculated and displayed
correctly, although they are illegal on the 8086.  Illegal offsets will
have more than four digits.

The segment register values are initialized as indicated in the file
header (for .EXE files) or to zero (for other files or RAM).  The
disassembler has no way of determining the values which may be set
during execution.  For example, the initialization code for DeSmet C
programs resets DS to the same value as the initial SS before executing
main().

The assumed segment register values can be altered in two ways.  Any
segment register can be changed using the register menu reached by the
'R' command.  In addition, when the right arrow key is used to follow a
far call or jump, the new code segment value is loaded into the CS
register.  When the user specifies a new segment value on an A, B, C, D,
or G command, that value is used for subsequent displays but none of the
assumed segment register values is changed.

The segmentation models of the protected modes of the 80286 and 80386
are not supported.


ALIGNMENT

Dis86 will correctly disassemble code if started on the first byte of
an instruction.  If started in the middle of an instruction, it will
disassemble that instruction and perhaps several more incorrectly.  In
this case the disassembler is said to be out of alignment with the
object code.  The disassembler will tend to correct its alignment if it
continues long enough.  8086 instructions tend to be longer than, for
example, those for the 8080, so the disassembler will tend to stay out
of alignment for more instructions.  Generally speaking, the alignment
will be correct after the first half dozen lines.


SUMMARY

Here are all the letter commands:

A nnnn     ASCII data
B nnnn     byte data (hex)
C nnnn     code (disassembly)
D nnnn     data (hex and ASCII)
E          enter new data (follow with a hex expression for each new byte)

G nnnn     goto address nnnn
H          display file header information (.EXE files only)
O          change setup options
P          print disassembly listing to file
Q          quit to DOS

R          change segment register values
S          select new segment register
W          set bytes of data per line for A, B, and D formats
X          exchange current address (at top of screen) with top of stack
?          display help screens



EXAMPLE 1

In the examples, <left>, <right>, <up>, and <down> refer to the four
cursor keys (4, 6, 8, and 2 on the numeric pad, plus the four arrow keys
on the Z-100 keyboard).  <pg up> and <pg dn> refer to 9 and 3 on the
numeric pad.

To investigate the bootstrap code, type

    A>dis86 <ret>

and press

    <space>

to advance to the disassembly display (which will be the interrupt
vectors).  Next type

    c a ffff:0000 <ret>

(for Code format at the Address ffff:0000).  On an IBM, the ROM release
date and machine ID appear in the last 16 bytes of the ROM.  To see them,
type

    D <ret>

The release data is at addresses ffff:0005 - ffff:000c in ASCII.  The
machine ID is at ffff:000e.  Some of the possible values are:

    ff    IBM PC
    fe    IBM XT and Portable IBM PC
    fd    IBM PCjr
    fc    IBM AT
    2d    Compaq
    9a    Compaq-Plus

Return to code format by typing

    C <ret>

One of the instructions displayed will almost certainly be a jump.  If
so, press

    <down>

enough times to bring the jump to the top line, then

    <right>

to follow the jump.  Note that the previous addresses were pushed onto
the stack, as shown on the bottom line.  To return to the most recent
address, press

    <left>

To leave the disassembler, press

    Q


EXAMPLE 2

For a second example, let us disassemble the disassembler itself.
Begin by typing

    A>dis86 dis86.exe <ret>

Note the header information, including the entry point of 0000:0000 and
the initial stack location of approximately 09e0:9eb8.  Proceed to the
disassembly screen by typing

    <space>

The disassembler starts in C (code) format at the entry point, which is
a jump to the initialization code.  To follow the jump, type

    <right>

One of the early instructions in the initialization code refers to the
first location in the stack segment.  Bring this location to the top of
the screen by typing

    <pg dn> <down> <down>

and follow the reference by typing

    <right>

Since it was a data reference, the disassembler automatically switched
to D (data) format.  Note that the two previous addresses have been
pushed onto the stack, as shown at the bottom of the screen.  Return to
the most recent one by typing

    <left>

The initialization code gets rather involved, but one of its functions
is to initialize DS to the same value as SS.  To reflect this, use the
R command:

    R

DS is the first register in the list, so you need only enter the
appropriate value:

    ss <ret> <space>

The code for the main program immediately followed the jump at
0000:0000.  To return there, type

    <left>

Send a copy of this screen to the file "printout" by typing

    P <ret> <ret> <ret>

To inspect the data segment, type

    A ds:0 <ret>

To display more characters on each line, use the W command:

    W 60 <ret>

Use the search command to find one of the messages:

    G S T hime <ret>

This string won't be found.  To correct the spelling to "home" and try
again, type

    G S T <right> o <del> <ret>

Once again, leave the disassembler by pressing

    Q